Field of the Invention
This invention relates generally to a system and method for estimating the conductivity of a cooling fluid flowing in a fuel cell system and, more particularly, to a system and method for estimating the conductivity of a cooling fluid flowing in a fuel cell system, where the method measures a positive to chassis isolation resistance at a high system power level, measures the positive to chassis isolation resistance at a low system power level, measures fuel cell stack voltage and battery voltage at the two power levels, and uses the two positive to chassis isolation resistances and the voltages to identify a stack coolant positive to chassis isolation resistance.
Discussion of the Related Art
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type for vehicles, and generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer, where the catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. A fuel cell stack typically includes a series of flow field or bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source in addition to the fuel cell stack, such as a high voltage DC battery or an ultracapacitor. A DC/DC converter is typically employed to match the stack voltage to the voltage of the battery. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to an electrical traction motor through a DC high voltage electrical bus for vehicle operation. The battery provides supplemental power to the electrical bus during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power, however, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge the battery or ultracapacitor at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery or ultracapacitor.
It is necessary to provide control algorithms on a fuel cell hybrid vehicle to determine how much power will be provided by the fuel cell stack and how much power will be provided by the battery in response to a driver power request and under all vehicle operating conditions. It is desirable to optimize the power distribution provided by the fuel cell stack and the battery so that the amount of hydrogen used to operate the vehicle is minimized. In other words, it is desirable to operate the fuel cell system in the most efficient manner that allows the vehicle to travel the farthest distance using the least amount of hydrogen. The battery must be operated within a defined state-of-charge (SOC) range, where the control algorithms typically provide a SOC set-point to which the battery charge and discharge is controlled based on that set-point.
In order to provide safe operation of a fuel cell hybrid vehicle, all high voltage parts of the electrical system on the vehicle need to be electrically isolated from the vehicle chassis. One way of providing high voltage isolation is to maximize one or more of the isolation resistances that limit the current flow to the chassis from a high voltage source, as is well understood by those skilled in the art. The loss of high voltage isolation between the vehicle electrical system and the vehicle chassis must be detectable during vehicle operation. When a high voltage isolation fault is detected, the isolation fault detection system will take suitable action, such as shutting down the system or providing a warning light to the vehicle operator.
The cooling fluid flowing through cooling channels in the fuel cell stack to cool the bipolar plates could provide an electrical connection between the fuel cell stack and the vehicle chassis, such as at the cooling system radiator. Thus, the cooling fluid is designed to have a low conductivity. However, over time, impurities and other contaminants enter the cooling fluid as a result of age and wear on the system, where those contaminants increase the ions in the cooling fluid making it more conductive. Also, as the cooling fluid is continually heated and cooled, it breaks down, also increasing its conductivity. Therefore, the cooling fluid needs to be periodically replaced so that it is not a cause of loss of high voltage isolation. Detecting for the loss of high voltage isolation may indicate that loss of isolation has occurred that could be caused by an increased conductivity of the cooling fluid before its next scheduled replacement. When loss of high voltage isolation is detected, a service technician will typically have to isolate components from the high voltage bus to determine the cause of the isolation fault, which is time consuming and labor intensive. If the technician knew that it was the conductivity of the cooling fluid causing the fault, then the cooling fluid could be replaced without having to test all of the other components in the high voltage system.